1. Field of the Invention
The present invention relates to a bioelectrical impedance measuring apparatus that can be mounted on an implantable medical instrument.
Priority is claimed on Japanese Patent Application No. 2011-018201, filed Jan. 31, 2011, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
A method disclosed in Japanese Unexamined Patent Application, First Publication No. 2008-168120 is well known as a method of precisely calculating a cardiac output and a pulmonary artery wedge pressure of a living body. FIG. 11 shows a configuration of a cardiac output monitor system disclosed in Japanese Unexamined Patent Application, First Publication No. 2008-168120. Hereinafter, a schematic operation of the cardiac output monitor system will be described with reference to FIG. 11. In addition, the configuration disclosed in Japanese Unexamined Patent Application, First Publication No. 2008-168120 is shown in FIG. 11 with convenient modifications of the following description added.
The cardiac output monitor system calculates a cardiac output and a pulmonary artery wedge pressure using an impedance signal that can be obtained by applying an alternate current to the heart. An extracting unit 11 extracts a minimum impedance signal Zmin, a maximum impedance signal Zmax, and an impedance average value signal Zmean based on an impedance signal received from a receiving unit 10.
A solid tissue derived impedance estimating unit 12 estimates a solid tissue derived impedance Zs based on a data set of a plurality of cardiac cycles including a maximum value and a minimum value in one cardiac cycle of an impedance signal that can be obtained within a predetermined time after injecting a hypertonic salt solution during a pulmonary circulation. A cardiac output calculating unit 13 precisely calculates a cardiac output using the following equation (A):CO=k·(1/(Zmin−Zs)−1/(Zmax−Zs))·HR  (A)where CO: cardiac output, k: correction factor, and HR: heart rate.
A pulmonary artery wedge pressure calculating unit 14 precisely calculates a pulmonary artery wedge pressure using the following equation (B):PAWP=A×C/(Zmean−Zs)−CO×B  (B)where PAWP: pulmonary artery wedge pressure, and A, B, C: correction factors.
In general, the value of the impedance average value signal Zmean of the heart is known to be about 500Ω, the value of the minimum impedance signal Zmin is known to be 500 Ω−5Ω, and a value of the maximum impedance signal Zmax is known to be about 500 Ω+5Ω. That is, the difference between the impedance average value signal Zmean and the minimum impedance signal Zmin or the maximum impedance signal Zmax is about ±1% with respect to the impedance average value signal of the heart.
When a voltage corresponding to an impedance signal of the heart is to be applied to an analog-digital (AD) converter to perform the calculation disclosed in the above equations (A) and (B), in order to detect an impedance-varying signal Zac, which is a differential between the impedance average value signal Zmean, the minimum impedance signal Zmin, and the maximum impedance signal Zmax of the heart caused by the cardiac output, a large number of effective bits is needed by the AD converter. This is shown in FIG. 12. In addition, an impedance average voltage Vmean, an impedance-varying voltage Vac, an impedance maximum voltage Vman, and an impedance minimum voltage Vmin in FIG. 12 are voltage signals corresponding to the impedance average value signal Zmean, the impedance-varying signal Zac, the maximum impedance signal Zmax, and the minimum impedance signal Zmin, respectively. Further, 1 LSB is a minimum resolution of the AD converter.
The following equation (C) is a relational expression representing power consumption Pd of the AD converter. Here, fc: sampling frequency, and ENOB: the number of effective bits. Equation (C) represents that the power consumption Pd of the AD converter is in proportion to a product of the sampling frequency fc and 2ENOB.Pd∝fC×2ENOB  (C)
As can be seen from equation (C), the AD converter having a large number of effective bits requires a large amount of power. This means that the cardiac output monitor system, on which the AD converter is mounted, also requires a large amount of power. Thus, the time that an implantable medical instrument can be implanted may be shortened, on which the cardiac output monitor system is mounted.